One input is routed to the output via two separate paths: One VCA with zero to positive unity gain (approx.), and one manually adjustable path with zero to rather high negative gain. The output voltage is clipped by a set of zener diodes, so the module can also act as Overdrive / Distortion device.

Some of you may recognize a korg MS20 filter in this one which is one of the designs I experimented with
and a couple of circuits based on that or similar too. As you can see there is another variable zener circuit
in there which has the same purpose as the zeners in the DUAL VCA.
I did want resonance controll but no tweeting self oscillation, so adjusted the values for that. But with
resonance set at max and a high amplitude input signal it still does oscillate sometimes, it also acts a bit weird with
resonance maxed and the filter turned down, but I decided to call that a bonus feature.

This wave shaper needs an input saw that goes from Vss to (ideally) Vdd. It works well with the 4069 VCO designed by René Schmitz. Any saw will work as long as the wave is centered halfway between the rails. It works as follows: The upper set of 2 linear mode gates provide an inverted and noninverted copy of the saw. The 3 logic gates at the bottom provide a sort of comparator that switches at halfway between the rails. The 4007 provides 2 P-MOSFETs that alternately select the inverted or noninverted saw signal depending on where the saw is in it's cycle. This causes the ramp to reverse directions every half cycle. Any imperfection in the saw will show up in the resulting output. The linear mode gate furthest to the right provides soft clipping that turns the triangle wave into something like a sine wave. It sounds like it has fewer harmonics than the triangle.

VCA1 has both linear and exponential inputs. The linear CV input is hardwired to the ADSR1 output. The exponential CV input is hard wired to the MIDI Velocity control voltage. This is how touch sensitivity is produced.

Voltage Controled Amplifier by ACX Synth ,page dedicated to this THAT 2181 based VCA http://www.acxsynth.com/VCA/vcafr.htm ,along with bill of materials and details on construction and performance .From there the brief description :

THAT the company specializes in high-end audio circuits. Among these, there is a VCA interesting is the 2180. It requires no adjustment and provides excellent performance. The dynamic range is greater than 120 dB distortion 0.0025% (model A). This circuit is however quite expensive. For this module, I simply resume the manufacturer's datasheet. My work is limited to the design of the PCB and the design of the control system voltage.

This circuit is logarithmic response which indicates that the gain in dB is linear with respect to the input voltage.

Looking at the schematic, you'll see it's a triangle core like the Buchla 259 core we discussed in class, except a CA3080 is used instead of the "roll your own" 4-transistor OTA Buchla uses. The HA2500 op ap is acting as the comparator, and the FET and BJT at the output of the 3080 are acting as a buffer. I think the weird configuration of Q5 and Q6 is acting as a voltage clamp. It appears to have been intended to be some sort of plug-in module for a bigger circuit.

In my version, as it is published here, I have replaced the differential buffer amp (which is a true piece of art in the original, using a FET pair, a BJT pair and an opamp) with an instrumentation amp made of BiFET opamps.
The compander circuit, built around a CMOS transistor array, is like in the original, and it surely contributes to the specific sound. (It's discussed on my Storm Tide Flanger page.)

This VCF has been heavily influenced by Jürgen Haibles SSM 2040 filter clone. I constructed it using mostly SMD parts, hence the name. The circuit differs from Jürgens, in a few details. I operate the filter core with different supplies. (+15V,+5V,0V) Thus I didn't need to lower the ground of the expo convertor. I omitted the voltage controlled resonance, because it is easy enough to get. If you have a spare VCA you can patch it if you need it. I'll maybe add it later, when I do a board revision.

Real Ring Modulator and Modem by Ken Stone ,for more info on this module ,on construction ,bill of materials and pcb's visit http://www.cgs.synth.net/ from there the brief description and an idea on how to combine ring modulators as Modem :

Four quadrant multipliers have more or less replaced ring modulators in synthesizers, even though they still bear the label "ring modulator". The distortion in these is lower than that of a true diode ring modulator, because the diode voltage drops have been eliminated. This leads to better specifications, though in a noise maker, this is not always desirable.

This is something you can try if you have two Real Ring Modulators, or with a little variation, any two four quadrant multipliers.

In radio work, modulation is only the first step. Before the signal can be heard at the receiving end, it must first be demodulated. The wiring diagram below shows a simple system that more-or-less replicates this, but without all the extra gear associated with radio transmission and reception.

Basically we need to feed the same carrier into two Real Ring Modulators, the second modulator being used in reverse to demodulate the signal that was modulated in the first. In a perfect world this would result in an identical signal to what was being fed in, which of course would make the exercise pointless in a synthesizer. Fortunately, Real Rings Modulators introduce distortion, so some interesting sounds can be achieved.

When the two Real Ring Modulators are interconnected, it is possible to not use any buffer in between them, though in this case, (allowing for some variation in wiring,) you would need to connect the outputs together and take your new output from the input of the second. This effectively reverses the phase of the second carrier. Alternatively you can install a switch as shown.

As soon as a buffer or modifier (for example a filter) is added between the two modulators, the correct phase will need to be determined by experiment, as some modules will invert the signal.

This is my first analog circuit. It was inspired by VAZ software synth's distortion module,
which had this wave wrap feature I considered cool. So after an evening of EWB simulations
I came up with this circuit.

SEKU is a two-channel 16-step analog sequencer.Heart of SEKU is CMOS 4520 dual synchronous divide-by-16 counter. RESET-in-jack must be normalised to ground when not used (16-step mode). Length of the sequence can be set to N by patching PULSE OUT from N+1 step to RESET IN jack. When no clock is connected to CLK IN sequencer can be manually stepped from STEP button.

CMOS 4067 1-of-16 analog switch is used for GATE, CV and PULSE outputs. Inhibit-pin of GATE OUT 4067-switch is clocked at the same frequency as the 4520 with 4001 nor-gate turning each GATE OUT off before turning the next GATE on. You can mute any one of the 1-16 steps with the STEP MUTE switch.

I have used slider potentiometers for STEP CV adjusting so that it is easier to draw waveforms when SEKU is used as an oscillator. I havent buffered the CV and GATE outputs with op amps in order to keep the power supply simple, just +5V to ground. I have also an adjustable power supply so I can adjust the octave range of SEKU by adjusting the supply voltage (+3 to +5 V or from 3 to 5 octaves in 1V/oct systems)

something i have been playing with this last week
vactrol type isnt fussy i am using NSL32 but could just as well be
vtl5c3, A slow closing vactrol like a vtl5c4 could be interesting also
the P2 connector goes to a B100k pot pin 2 is the wiper just reverse 1 and 3 if left / right is wrong way round . note with the pot in the centre and no cv input there is no output.or at least very little
CV input can be from an LFO output +5 to -5 volts though going to the rails will not hurt anything.
T1 and T2 adjust the sensitivity for the vactrols you can get them pretty good just by watching the panel leds PD-L / PD-R while adjusting P2 for the supplied voltage though the actual vactrol response is slightly different to normal leds so also use your ears
anyway hope someone can use this

While tuning my Oakley and other homer VCO synth modules I originally used to use and old £45 Maplin "Precision Gold" multimeter of 1990 vintage and use the surprisingly accurate frequency counter on there to set the VCOs to the A440 international tuning standard. However I'd seen a prototype Moog module in the booklet of the Switched On Bach CD box set which Bob Moog once made as a Christmas Present for Wendy Carlos. I thought it was time for a crystal controlled reference oscillator design. The requirements were standard crystal stability (easily good enough for analogue synths), sine wave output and extreme simplicity and cheapness.

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